Battery stack

ABSTRACT

A first end plate is disposed at one end, in a stacking direction, of a stacked body of secondary battery cells. A second end plate is disposed at the other end, in the stacking direction, of the stacked body of the secondary battery cells. A restraining member is joined to the first end plate and the second end plate. The restraining member applies, to the first end plate and the second end plate, restraint loads that sandwich and restrain the stacked body from both sides in the stacking direction. The first end plate is movable relative to the second end plate in the state where the restraining member is joined to both the first end plate and the second end plate.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-200861 filed onSep. 30, 2014 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a battery stack and, in particular, relates toa battery stack formed by stacking a plurality of battery cells eachhaving a current interrupt device.

2. Description of Related Art

Conventionally, there is proposed a battery pack in which unit batteriesare arranged side by side and integrated by fastening end platesdisposed at both ends in an arrangement direction of the unit batteriesby means of restraining bands (see, e.g. Japanese Patent ApplicationPublication No. 2001-68081 (JP 2001-68081 A)).

Further, there is proposed a battery module including a pair of endplates disposed at both ends of a battery unit in which a plurality ofbatteries are connected to each other, restrainers joined to the endplates, and joining units configured to fix one of the end plates to therestrainers while allowing joint positions therebetween to be changed(see, e.g. Japanese Patent Application Publication No. 2011-129509 (JP2011-129509 A)).

Further, there is proposed a battery module in which a battery issandwiched between a first restraining plate and a second restrainingplate having deformable portions and, when the pressure inside thebattery reaches a predetermined value, the deformable portions aredeformed to relieve the pressure (see, e.g. Japanese Patent ApplicationPublication No. 2013-114943 (JP 2013-114943 A)).

As a technique for interrupting a current of a lithium-ion battery atthe time of overcharging, the generation of gas due to an overchargeadditive contained in an electrolyte solution and a current interruptdevice (CID) have been widely used in combination thereof.

SUMMARY OF THE INVENTION

A battery stack according to an aspect of the invention includes aplurality of battery cells. Each battery cell includes a batteryelement; a housing receiving the battery element therein; an externalterminal disposed outside the housing; and a current interrupt device.The current interrupt device is configured to operate when an internalpressure of the housing is increased, thereby interrupting electricalconnection between the battery element and the external terminal. Theplurality of battery cells are stacked in one direction to form astacked body. The battery stack further includes a first end plate, asecond end plate, and a restraining member. The first end plate isdisposed at one end, in the one direction, of the stacked body of thebattery cells. The second end plate is disposed at the other end, in theone direction, of the stacked body of the battery cells. The restrainingmember is joined to the first end plate and the second end plate. Therestraining member is configured to apply, to the first end plate andthe second end plate, restraint loads that sandwich and restrain thestacked body from both sides in the one direction. The first end plateis movable relative to the second end plate in a state where therestraining member is joined to both the first end plate and the secondend plate.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a perspective view showing a secondary battery cell that formsa battery stack according to an embodiment of the invention;

FIG. 2 is a diagram for explaining a current interrupt device providedin the secondary battery cell shown in FIG. 1;

FIG. 3 is a side view showing a configuration of a battery stack of afirst embodiment;

FIG. 4 is a plan view showing a configuration of the battery stack ofthe first embodiment;

FIG. 5 is an exemplary diagram showing a configuration of a restrainingmember;

FIG. 6 is a side view showing a configuration of a battery stack of asecond embodiment;

FIG. 7 is a plan view showing a configuration of the battery stack ofthe second embodiment;

FIG. 8 is a diagram showing the results of evaluation tests of batterystacks of Examples 1 and 2 and Comparative Examples 1 and 2; and

FIG. 9 is a diagram showing the results of evaluation tests of batterystacks of Example 3 and Comparative Example 3.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinbelow, embodiments of the invention will be described withreference to the drawings. In the drawings, the same reference numeralsare assigned to the same or corresponding portions, thereby omittingduplicate description thereof.

First Embodiment

FIG. 1 is a perspective view showing a secondary battery cell 10 thatforms a battery stack according to an embodiment of the invention.Referring to FIG. 1, a plurality of secondary battery cells 10 in thisembodiment are combined in series to form a battery stack, which isinstalled in a hybrid vehicle. The battery stack serves as a powersource of the hybrid vehicle along with an internal combustion enginesuch as a gasoline engine or a diesel engine.

The secondary battery cell 10 includes a battery element B, a case 15, asealing member 16, a positive electrode terminal 11, and a negativeelectrode terminal 12. The battery element B is formed by stackingpositive and negative electrode plates with a separator interposedtherebetween. The case 15 has a generally rectangular parallelepipedshape that is open in one direction. The sealing member 16 has a flatplate shape that is generally rectangular in plan view. The sealingmember 16 is provided so as to close the opening of the case 15. Thecase 15 and the sealing member 16 are made of a conductive material suchas a metal typified by aluminum.

The case 15 and the sealing member 16 jointly define a sealed space. Thecase 15 and the sealing member 16 jointly form an outer package of thesecondary battery cell 10. The case 15 and the sealing member 16 jointlyform a housing that receives the battery element B therein. The housingof the secondary battery cell 10 has a generally rectangularparallelepiped shape. In the housing, the battery element B is placedalong with an electrolyte solution. The electrolyte solution contains agas generating agent (overcharge additive) that can be decomposed togenerate a gas when a predetermined battery voltage is exceeded.

The positive electrode terminal 11 and the negative electrode terminal12 are attached to the sealing member 16. The positive electrodeterminal 11 and the negative electrode terminal 12 are provided so as toprotrude from the housing of the secondary battery cell 10. The positiveelectrode terminal 11 and the negative electrode terminal 12 aredisposed outside the housing of the secondary battery cell 10. Thepositive electrode terminal 11 and the negative electrode terminal 12form external terminals of the secondary battery cell 10.

The secondary battery cell 10 includes a mechanism (current interruptdevice 100) configured to interrupt the flow of current between thebattery element B and the external terminal when the internal pressureof the case 15 is increased. The current interrupt device 100 isprovided to at least one of the positive electrode terminal 11 and thenegative electrode terminal 12.

FIG. 2 is a diagram for explaining the current interrupt device 100provided in the secondary battery cell 10. The current interrupt device100 is mounted in the secondary battery cell 10 shown in FIG. 1.

The current interrupt device 100 is a pressure-type current interruptdevice and is used in a sealed-type battery. Specifically, the currentinterrupt device 100 operates when the battery internal pressure (thepressure in the housing formed by the case 15 and the sealing member 16)is increased, thereby interrupting a current between the battery elementB and the external terminal (the positive electrode terminal 11 or thenegative electrode terminal 12).

As shown in FIG. 2, the current interrupt device 100 includes aninsulator 180, a conductive member 130, an inversion plate 120, acurrent collector terminal (current collector plate) 101, and a holdermember 160.

A terminal plate 190 shown in FIG. 2 is made of a conductive materialand electrically connected to the external terminal (the positiveelectrode terminal 11 or the negative electrode terminal 12) shown inFIG. 1. The insulator 180 is made of an insulating material. Theinsulator 180 is interposed between the sealing member 16 and theterminal plate 190. The insulator 180 provides electrical insulationbetween the sealing member 16 and the terminal plate 190.

The conductive member 130 is made of a conductive material such ascopper or aluminum. The sealing member 16 is formed with a through hole141 penetrating the flat plate-shaped sealing member 16 in its thicknessdirection. The conductive member 130 passes through the through hole141. The conductive member 130 is fitted into the through hole 141. Theconductive member 130 is connected to the terminal plate 190 outside thehousing of the secondary battery cell 10 and connected to the inversionplate 120 inside the housing of the secondary battery cell 10. Theconductive member 130 establishes electrical connection between theterminal plate 190 and the inversion plate 120. The conductive member130 has a shape whose diameter increases in the housing of the secondarybattery cell 10, and the inversion plate 120 is attached to theincreased-diameter portion of the conductive member 130.

The inversion plate 120 and the current collector terminal 101 aredisposed in the housing of the secondary battery cell 10. The inversionplate 120 is made of a conductive material. The inversion plate 120 isdisposed between the conductive member 130 and the current collectorterminal 101. The inversion plate 120 is fixed to the conductive member130 and the current collector terminal 101, for example, by welding. Theinversion plate 120 establishes electrical connection between theconductive member 130 and the current collector terminal 101. Normally,the inversion plate 120 is concave on the conductive member 130 side andconvex on the current collector terminal 101 side.

The inversion plate 120 has a generally disk shape. The inversion plate120 has a pressure sensitive surface 121 at its central portion. Thecurrent collector terminal 101 has a thin portion 111 at its centralportion. Normally, the pressure sensitive surface 121 is fixed to thethin portion 111. The edge portion of the inversion plate 120 is fixedto the conductive member 130.

The holder member 160 is provided in the housing of the secondarybattery cell 10. The holder member 160 is provided directly under thesealing member 16. The holder member 160 is disposed so as to besandwiched between the sealing member 16 and the conductive member 130.The outer peripheral portion of the conductive member 130 is in contactwith the holder member 160. The holder member 160 holds the currentcollector terminal 101. The holder member 160 also serves to seal thethrough hole 141 formed in the sealing member 16.

The holder member 160 has a caulking portion 162. The current collectorterminal 101 is caulked by the caulking portion 162 so as to be held bythe holder member 160.

The current collector terminal 101 is connected to the electrode of thebattery element B shown in FIG. 1. Before the current interrupt device100 starts to operate, electric power (current) from the battery elementB flows through the current collector terminal 101, the inversion plate120, the conductive member 130, the terminal plate 190, and the externalterminal (the positive electrode terminal 11 or the negative electrodeterminal 12) in this order. Consequently, the electric power is suppliedfrom the secondary battery cell 10 to the outside. When charging thesecondary battery cell 10, a current flows in a direction opposite tothe above.

When the internal pressure (of the housing formed by the case 15 and thesealing member 16) of the secondary battery cell 10 is increased, thepressure sensitive surface 121 is pressed by a gas in the housing. Therigidity of the thin portion 111 of the current collector terminal 101is low compared to the other portions. Therefore, when the internalpressure of the housing is increased to a predetermined pressure(working pressure) or more, breakage occurs at the thin portion 111 ofthe current collector terminal 101 so that the inversion plate 120 isdeformed in a direction away from the current collector terminal 101.More specifically, the inversion plate 120 is inverted so as to beconvex on the conductive member 130 side and concave on the currentcollector terminal 101 side.

When the inversion plate 120 is deformed due to the breakage of thewelded portion between the thin portion 111 of the current collectorterminal 101 and the pressure sensitive surface 121 of the inversionplate 120, the conductive member 130 and the current collector terminal101 are isolated from each other. By this theory, electrical connectionbetween the battery element B and the external terminal is interruptedso that a current flowing between the battery element B and the externalterminal is interrupted.

FIG. 3 is a side view showing a configuration of a battery stack of afirst embodiment. FIG. 4 is a plan view showing a configuration of thebattery stack of the first embodiment. The battery stack shown in FIGS.3 and 4 is formed by connecting a plurality of secondary battery cells10 of which one has been described with reference to FIGS. 1 and 2. Inthe battery stack, the secondary battery cells 10 are stacked in aleft-right direction in the figures, thereby forming a stacked body. Thesecondary battery cells 10 are arranged so that side surfaces, havingthe largest area, of the respective housings face each other.

The secondary battery cells 10 are connected to each other by bus bars13. The secondary battery cells 10 are stacked in a state wheredirections of the secondary battery cells 10 are alternately reversed sothat the positive electrode terminal 11 and the negative electrodeterminal 12 adjoin each other between the adjacent two secondary batterycells 10. The positive electrode terminal 11 of each secondary batterycell 10 is disposed adjacent to the negative electrode terminal 12 ofthe adjacent secondary battery cell 10, and the positive electrodeterminal 11 and the negative electrode terminal 12 are connected to eachother by the bus bar 13. Consequently, the secondary battery cells 10are connected in series.

End plates 30 and 40 are disposed at both ends, in the stackingdirection (left-right direction in FIGS. 3 and 4) of the secondarybattery cells 10, of the stacked body. The end plate 30 is disposed atone end in the stacking direction with respect to the stacked body ofthe secondary battery cells 10. The end plate 40 is disposed at theother end in the stacking direction with respect to the stacked body ofthe secondary battery cells 10. The end plates 30 and 40 are disposed sothat their main surfaces face each other. The secondary battery cells 10are arranged between the end plates 30 and 40. The end plates 30 and 40sandwich therebetween the stacked body of the secondary battery cells 10from both sides in the stacking direction of the secondary battery cells10.

Intervening members 21 and 22 are disposed between the adjacentsecondary battery cells 10, 10. Since the intervening members 21 and 22are interposed between the adjacent secondary battery cells 10, thesecondary battery cells 10 are respectively arranged at a distance fromeach other. The intervening member 21 is made of an insulating materialand ensures insulation between the adjacent two secondary battery cells10. The intervening member 22 forms a gap between the intervening member21 and the secondary battery cell 10, thereby facilitating cooling ofthe secondary battery cell 10.

The end plates 30 and 40 are coupled to each other by restrainingmembers 50. Each restraining member 50 extends in the stacking directionof the secondary battery cells 10 of the stacked body from the end plate30 to the end plate 40. As shown in FIG. 3, the restraining members 50are provided in plural in a height direction (up-down direction in FIG.3) of the secondary battery cells 10 and the end plates 30 and 40. Asshown in FIG. 4, the restraining members 50 are provided on both sidesurfaces of the end plates 30 and 40.

The stacked body of the secondary battery cells 10 therearound isrestrained by the end plates 30 and 40 and the restraining members 50.The stacked body of the secondary battery cells 10 is pressed in thestacking direction. The restraining members 50 apply, to the end plates30 and 40, restraint loads that sandwich and restrain the stacked bodyof the secondary battery cells 10 from both sides in the stackingdirection.

FIG. 5 is an exemplary diagram showing a configuration of therestraining member 50. As shown in FIG. 5, the restraining member 50 hasan elongated plate shape. The restraining member 50 is formed with acircular hole 51 and an elongated hole 52. The circular hole 51 and theelongated hole 52 are each formed as a through hole penetrating therestraining member 50 in its thickness direction. The circular hole 51is formed near one end of the restraining member 50. The elongated hole52 is formed near the other end of the restraining member 50. Theelongated hole 52 has a shape extending in an extending direction of therestraining member 50.

The restraining member 50 is fixed to the end plates 30 and 40 by fixingmembers 61 such as pins or screws. The fixing member 61 passes throughthe elongated hole 52 formed in the restraining member 50 and is fixedto the end plate 30. The fixing member 61 passes through the circularhole 51 formed in the restraining member 50 and is fixed to the endplate 40. The circular hole 51 is formed at a portion where therestraining member 50 is joined to the end plate 40. The elongated hole52 is formed at a portion where the restraining member 50 is joined tothe end plate 30.

In the state where the restraining member 50 is joined to the end plates30 and 40 by the fixing members 61, the circular hole 51 is covered inits entirety by the fixing member 61 in side view of the battery stackshown in FIG. 3. On the other hand, the elongated hole 52 is coveredonly partially by the fixing member 61 and, as shown in FIG. 3, part ofthe elongated hole 52 is visible from the side.

The restraining member 50 extends in the stacking direction of thesecondary battery cells 10 of the stacked body. The elongated hole 52extends in the extending direction of the restraining member 50.Therefore, in the state where the restraining member 50 is joined to theend plates 30 and 40, the elongated hole 52 extends in the stackingdirection of the secondary battery cells 10 of the stacked body.

The battery stack shown in FIGS. 3 and 4 can be manufactured as follows.First, the secondary battery cells 10 of which one is shown in FIG. 1are prepared. The secondary battery cells 10 are stacked in onedirection, then the end plates 30 and 40 are disposed at both ends ofthe stacked body, and then the intervening members 21 and 22 aredisposed between the adjacent secondary battery cells 10. Using a knownpressing jig or apparatus, a pressing force in the stacking direction ofthe secondary battery cells 10 of the stacked body is applied to the endplates 30 and 40.

In the state where the stacked body is pressed from both sides, therestraining members 50 are joined to the end plates 30 and 40. Morespecifically, in the state where the restraining members 50 are disposedon the side surfaces of the end plates 30 and 40, the fixing members 61are inserted through the circular holes 51 of the restraining members 50and fixed to the end plate 40 and, likewise, the fixing members 61 areinserted through the elongated holes 52 of the restraining members 50and fixed to the end plate 30.

In this manner, restraint loads that sandwich the stacked body of thesecondary battery cells 10 from both sides in the stacking direction areapplied to the stacked body of the secondary battery cells 10, therebyobtaining the battery stack in which the secondary battery cells 10 arerestrained.

In the battery stack thus configured, each secondary battery cell 10includes the current interrupt device 100 in the housing as describedabove. Further, the gas generating agent (overcharge additive) isprovided in the housing of the secondary battery cell 10. At the time ofovercharging, the gas generating agent generates a gas to increase theinternal pressure of the housing, thereby operating the currentinterrupt device 100 to interrupt a current. Consequently, the secondarybattery cell 10 is protected against overcharging.

In order to reliably operate the current interrupt device 100 at thetime of overcharging, a sufficient amount of gas generation is required.However, if degassing from the battery element B is insufficient, theelectrolyte solution is forced out due to remaining gas so that reactionis inhibited, leading to a reduction in gas generation. As a result, theamount of gas generation becomes insufficient.

Therefore, in the battery stack of this embodiment, the elongated hole52 is formed in each restraining member 50. The elongated hole 52extends in the extending direction of the restraining member 50. In thestate where the restraining members 50 are attached to the end plates 30and 40, the elongated holes 52 each extend in the stacking direction ofthe secondary battery cells 10. The end plate 30 is provided so as to beable to change its relative position with respect to the restrainingmembers 50 along the elongated holes 52. In the state where therestraining members 50 are joined to both the end plates 30 and 40, theend plate 30 is movable relative to the end plate 40 in the stackingdirection of the secondary battery cells 10.

Consequently, the stacked body of the secondary battery cells 10 isconfigured such that its length in the stacking direction is variable inthe state where it is restrained from both sides by the end plates 30and 40. Accordingly, each secondary battery cell 10 is allowed toincrease its dimension in its thickness direction. When a gas isgenerated in each secondary battery cell 10 to increase the internalpressure at the time of overcharging, since each secondary battery cell10 can be expanded in its thickness direction, degassing from thebattery element B is facilitated.

With the configuration described above, at the time of overcharging, asufficient amount of gas is obtained in the housing of each secondarybattery cell 10 so that it is possible to reliably operate the currentinterrupt device 100.

Second Embodiment

FIG. 6 is a side view showing a configuration of a battery stack of asecond embodiment. FIG. 7 is a plan view showing a configuration of thebattery stack of the second embodiment. The battery stack of the secondembodiment shown in FIGS. 6 and 7 differs from the battery stack of thefirst embodiment in a configuration of a restraining member 50.

Specifically, restraining members 50 of the second embodiment each havean extending portion 53 extending in a stacking direction of secondarybattery cells 10 and engaging portions 54 provided at both ends of theextending portion 53 and engaging with end plates 30 and 40. Theengaging portions 54 are each in contact with a main surface, on theside not facing the secondary battery cell 10, of the end plate 30 or40. The engaging portions 54 are respectively fixed to the end plates 30and 40 using fixing members not shown in FIGS. 6 and 7.

The extending portion 53 and the engaging portions 54 are each formed bya flat plate-shaped member. The restraining member 50 may be formed bymachining a single plate-shaped member or may be formed by joiningtogether plate-shaped members that respectively form the extendingportion 53 and the engaging portions 54.

The restraining members 50 of the second embodiment apply restraintloads to a structure, in which a stacked body of the secondary batterycells 10 is sandwiched from both sides by the end plates 30 and 40, bysandwiching the structure from both ends.

In the battery stack of the second embodiment thus configured, byproperly selecting the material and shape of the extending portions 53of the restraining members 50, the extending portions 53 are deformedwhen a force of the secondary battery cells 10 to expand in a thicknessdirection thereof is applied to the extending portions 53 at the time ofovercharging. Consequently, in the state where the restraining members50 are joined to both the end plates 30 and 40, the end plate 30 ismovable relative to the end plate 40 in the stacking direction of thesecondary battery cells 10.

Therefore, as in the first embodiment, since each secondary battery cell10 can be expanded in its thickness direction at the time ofovercharging, degassing from the battery element B is facilitated andthus a sufficient amount of gas is obtained in the housing of eachsecondary battery cell 10 so that it is possible to reliably operate thecurrent interrupt device 100.

Hereinbelow, Examples of the invention will be described. In thefollowing Examples, a test of increasing the internal pressure of thesecondary battery cell 10 and an overcharge test of overcharging thesecondary battery cell 10 were carried out for the battery stacksdescribed in the above embodiments.

Secondary battery cells 10 of two specifications, i.e. cellspecification A and cell specification B, were prepared. In thesecondary battery cell 10 of cell specification A, a positive electrodewas a ternary positive electrode, while a material of a negativeelectrode was graphite. As an overcharge additive, 2 wt %cyclohexylbenzene (CHB) was used. As a separator isolating positive andnegative electrode plates of a battery element B from each other andholding an electrolyte solution between the electrode plates, athree-layer separator of PP (polypropylene)/PE (polyethylene)/PP wasused. The external dimensions of the secondary battery cell 10 were setto 150 mm in a width direction, 26 mm in a thickness direction, and 90mm in a height direction. The capacity of the secondary battery cell 10was set to 30 Ah.

The thickness direction of the secondary battery cell 10 corresponds toa direction in which a plurality of secondary battery cells 10 arestacked (left-right direction in FIGS. 3 and 4). The height direction ofthe secondary battery cell 10 corresponds to an up-down direction inFIG. 3. The width direction of the secondary battery cell 10 correspondsto an up-down direction in FIG. 4. The width direction, the thicknessdirection, and the height direction of the secondary battery cell 10 arethree directions perpendicular to each other.

In the secondary battery cell 10 of cell specification B, a positiveelectrode was a ternary positive electrode, while a material of anegative electrode was graphite. As an overcharge additive, 2 wt %cyclohexylbenzene (CHB) was used. As a separator provided betweenpositive and negative electrode plates of a battery element B andholding an electrolyte solution, a three-layer separator of PP(polypropylene)/PE (polyethylene)/PP was used. The external dimensionsof the secondary battery cell 10 were set to 138 mm in a widthdirection, 13 mm in a thickness direction, and 63 mm in a heightdirection. The capacity of the secondary battery cell 10 was set to 4Ah.

The prepared secondary battery cells 10 were stacked to form a stackedbody and end plates 30 and 40 were respectively disposed at both ends ina stacking direction of the stacked body. In the state where a load of500 kgf was applied in the stacking direction of the stacked body,fixing members 61 (bolts) were fastened to the end plates 30 and 40 at 3N·m, thereby restraining a battery stack.

A test of increasing the internal pressure of the secondary battery cell10 was carried out by forming a hole in one of the secondary batterycells 10 included in the battery stack and applying a pressure of 0.3MPa to this secondary battery cell 10 from the outside. A change in thedimension (stack entire length) of the battery stack in the stackingdirection of the stacked body in this event was measured.

An overcharge test was carried out for the secondary battery cell 10attached with an internal pressure sensor under test conditions ofcharging condition 20A (charging to SOC (State of Charge) 145%) and testtemperature 25° C. An internal pressure increase of the secondarybattery cell 10 at the time of overcharging was measured and acomparison was made.

FIG. 8 is a diagram showing the results of evaluation tests of batterystacks of Examples 1 and 2 and Comparative Examples 1 and 2. In Examples1 and 2, the tests were carried out using battery stacks according tothe first embodiment. In Comparative Examples 1 and 2, the tests werecarried out using battery stacks including restraining members eachformed with, instead of the elongated hole 52 of the restraining member50 of the first embodiment, a circular hole like the circular hole 51 ofthe restraining member 50 of the first embodiment.

As shown in FIG. 8, in the case of Examples 1 and 2, the stack entirelength was increased by 0.2 mm when the secondary battery cell 10 wascompressed to 0.3 MPa. On the other hand, in the case of ComparativeExamples 1 and 2, the stack entire length was increased only by 0.04 mmwhen the secondary battery cell 10 was compressed to 0.3 MPa.

Further, as shown in FIG. 8, in the case of Examples 1 and 2, since thestack entire length was allowed to change at the time of an internalpressure increase of the secondary battery cell 10, internal pressureincreases of 1.1 MPa and 1.0 MPa were respectively achieved at the timeof overcharging. On the other hand, in the case of Comparative Examples1 and 2, the internal pressures were respectively increased only by 0.7MPa and 0.6 MPa at the time of overcharging.

FIG. 9 is a diagram showing the results of evaluation tests of batterystacks of Example 3 and Comparative Example 3. In Example 3 andComparative Example 3, the tests were carried out using battery stacksaccording to the second embodiment. In Example 3, the cross-sectionalarea of each of restraining members 50 was set to 4 mm². In ComparativeExample 3, the cross-sectional area of each of restraining members 50was set to 8 mm². The restraining members 50 of Example 3 wereconfigured to be smaller in rigidity and thus to be deformed more easilythan the restraining members 50 of Comparative Example 3.

As shown in FIG. 9, in the case of Example 3, the stack entire lengthwas increased by 0.1 mm when the secondary battery cell 10 wascompressed to 0.3 MPa. On the other hand, in the case of ComparativeExample 3, the stack entire length was increased only by 0.05 mm whenthe secondary battery cell 10 was compressed to 0.3 MPa.

Further, as shown in FIG. 9, in the case of Example 3, since the stackentire length was allowed to change at the time of an internal pressureincrease of the secondary battery cell 10, an internal pressure increaseof 1.1 MPa was achieved at the time of overcharging. On the other hand,in the case of Comparative Example 3, the internal pressure wasincreased only by 0.7 MPa at the time of overcharging.

Therefore, if the end plate 30 is configured to be movable relative tothe end plate 40 by forming the elongated holes 52 in the restrainingmembers 50 or forming the restraining members 50 to be easilydeformable, the stack entire length of the battery stack can beincreased at the time of overcharging, thereby allowing expansion ofeach secondary battery cell 10 in its thickness direction. As a result,it is possible to ensure a sufficient amount of gas generation at thetime of overcharging and thus to reliably operate the current interruptdevice 100.

While the embodiments and Examples of the invention have been describedabove, it is to be understood that the embodiments disclosed this timeare for illustrative purposes only and are not intended to limit theinvention in any aspect. It is intended that the scope of the inventionbe defined by the scope of the claims, not by the above description andthat equivalents of the scope of the claims and all changes within thescope of the claims be included in the scope of the invention.

What is claimed is:
 1. A battery stack comprising: a plurality ofbattery cells each including: a battery element; a housing in which thebattery element is housed; an external terminal disposed outside thehousing; and a current interrupt device configured to operate when aninternal pressure of the housing is increased, thereby interruptingelectrical connection between the battery element and the externalterminal, the plurality of battery cells stacked in one direction toform a stacked body, the battery stack further comprising: a first endplate disposed at one end, in the one direction, of the stacked body; asecond end plate disposed at the other end, in the one direction, of thestacked body; and a restraining member joined to the first end plate andthe second end plate and configured to apply, to the first end plate andthe second end plate, restraint loads that sandwich and restrain thestacked body from both sides in the one direction, wherein the first endplate is movable relative to the second end plate in a state where therestraining member is joined to both the first end plate and the secondend plate.
 2. The battery stack according to claim 1, wherein anelongated hole extending in the one direction is formed at a portion,joined to the first end plate, of the restraining member.
 3. The batterystack according to claim 2, wherein a fixing member passes through theelongated hole and is fixed to the first end plate.
 4. The battery stackaccording to claim 1, wherein the restraining member includes anextending portion extending in the one direction and configured to bedeformable; and engaging portions provided at both ends of the extendingportion and engaging with the first end plate and the second end plate.